Filtration Apparatus

00:01 Here now is a diagram explaining the filtration apparatus in the glomerulus. Down the bottom is a diagram illustrating a vessel, a fenestrated capillary that's shown here in orange.

00:19 The kidney has fenestrated endothelium in the glomerulus to enhance the chance of filtration, to enhance the ability for the plasma to leak out into Bowman's capsule. And when it leaks out, it passes through gaps between the podocyte and a molecular sieve I'll describe in a moment. Have a look at the podocyte.

00:45 It's colored yellow in the bottom diagram. The podocyte is like an octopus wrapping its tentacles all around the capillary. And as you see here when it wraps its processes around the capillary, it creates filtration slits. And then between the podocyte and the endothelium is the basal lamina. It's a combined basal lamina produced by both the podocyte and the endothelial cells.

01:16 That's a very important structure too. And you can see details of that in a higher magnification or a higher representation on the top part of the diagram in the little square, or probably rectangle. Now here's an electron micrograph showing the details of that filtration barrier. Let's orientate ourselves. Down the bottom is the lumen of the capillary in the glomerulus. It has got fenestrations or little gaps in the wall. So the plasma is going to move out through those fenestrations. And then it confronts this basal lamina, which I said was produced by both the podocyte and the endothelial cell. It's a fairly thick layer. It's a molecular sieve.

02:15 It prevents certain substances, molecules, from passing through it.

02:19 Most proteins, for instance, are too large to get through that sieve. It's anionic so it tends to repel negatively charged molecules. And then further filtration is achieved by substances passing through this molecular sieve then through the filtration slits created by those podocyte processes, and there's also a very very thin molecular diaphragm there. It also participates in the filtration process. Finally, the filtrate passes into the luminal space or Bowman's space.

03:02 And that's the urinary space. And up on the top, you can see part of the cell body of one of these podocytes. So that's the filtration apparatus, and filtration is achieved by the hydrostatic pressure created by the efferent arteriole having a smaller diameter than the larger afferent arteriole. And this pressure here created across the glomerulus achieves the pressure required to drive that plasma filtrate through the molecular sieve into Bowman's space.